1. Field of the Invention
The present invention relates to a device for controlling pressure of brake fluid having a function of multiplying a brake force. More particularly, the present invention relates to a device for controlling pressure of brake fluid by which a function of multiplying a brake force, the multiplying factor of which is high, can be exhibited in the case of a normal operation and a braking force higher than that of a conventional device for controlling pressure of brake fluid can be obtained even in the case of a power failure of the boosting device.
2. Description of the Related Art
In general, inexperienced drivers and even experienced drivers, who have been seized with a panic, can not put on the brakes strongly, and it is impossible for the drivers to put on the brakes strongly over a long period of time. For the above reasons, automobile engineers have been developing a device for controlling pressure of brake fluid including an anti-lock control system by which the brake performance can be exhibited at the maximum.
In this connection, in the device for controlling pressure of brake fluid provided with a vacuum booster or hydraulic pressure booster, when a large-capacity booster is adopted to obtain a brake force of high intensity so as to meet the above requirements, the sensitivity of the booster is increased too high when a driver puts on the brakes weakly or when an automobile is running in a low deceleration range. Therefore, problems about safety may be caused. That is, as shown in FIG. 5, when the boosting function is simply enhanced in the brake device provided with a conventional booster, brake fluid pressure Pb is increased too high compared with brake fluid pressure Pa in the case of the conventional brake device when the driver puts on the brakes weakly, in other words, the sensitivity is too high. Therefore, the same problems as those caused in the case of suddenly applying the brakes are encountered. In the case of a failure of a booster, the boosting function of which is enhanced, it becomes impossible to generate a sufficiently high brake force, which causes problems about safety.
In order to solve the above problems, for example, there is disclosed a brake device in Japanese Patent Publication No. Hei. 8-58560.
The brake device disclosed in Japanese Unexamined Patent Publication No. 8-58560 comprises: a master cylinder to give brake fluid pressure to the wheel cylinder; a brake fluid pressure booster for multiplying a force given to the brake pedal by a predetermined ratio of multiplication so as to transmit the multiplied pressure to the master cylinder, arranged between the brake pedal and the master cylinder; a pressure intensifying mechanism to intensify the brake fluid pressure of the master cylinder so as to transmit the intensified pressure to the wheel cylinder, arranged in the brake fluid passage communicating the wheel cylinder with the master cylinder; and a control valve to operate the pressure intensifying mechanism in the case of a failure of the above brake fluid pressure booster. In the above brake device, when the brake fluid booster is out of order because of a failure of a brake fluid pressure source, the control valve acts immediately and conducts intensifying the brake force. Therefore, it is possible to prevent a delay of braking operation.
However, in the above intensifying mechanism, the following problems may be encountered. Both the brake fluid pressure booster and the intensifying mechanism are arranged in one system. Accordingly, the intensifying mechanism can not operate in the case of a normal operation. Therefore, it is impossible for the above intensifying mechanism to solve the problems caused when a driver is seized with panic and puts on the brakes too weakly in case of emergency. Further, the mechanism to make up for the failure of brake power is complicated, and the equipment cost is raised.